I agree that a full four months (including research and planning) should really be adequate for a Group Build. However, I also think it makes sense to have all GBs finishing at the end of a weekend, rather than mid-week - two or three extra days shouldn't cause any major hassles - and of course most people have more free time on a weekend, when those last minute niggles can be corrected, or major disasters rectified.
BUT - given that all concerned agree (to have GBs finish on the weekend following the end of the relevant month, if the month ends on a weekday), then I feel that, as has been discussed, that should be the only extension, otherwise we'll get back to the situation we've seen in the past, with GBs dragging on for an extra week, then another, and so on.